The invention relates to a method and apparatus for the separation of liquid from a suspension of finely comminuted fiber material and the liquid. In the pulpmaking field there are many situations when liquid is separated from an established flow of a fiber material suspension, such as in thickening of the suspension, and in diffusing (i.e., chemical treatment of the suspension fibers as by bleaching, or washing of the suspension). Exemplary prior art methods and apparatus for accomplishing this are shown in U.S. Pat. Nos. 3,372,087 and 3,563,891, and in copending application Ser. No. 621,561, filed Oct. 10, 1975, at present pending. In such prior art arrangements, a screen member disposed in a flow of fiber material suspended in liquid is reciprocated back and forth along the direction of flow, reciprocation in the direction with the flow being relatively slow (matching the speed of suspension flow) so that a fiber mat is built up on the screen member face, and movement of the screen member opposite the direction of suspension flow being relatively rapid so that the fiber mat is separated from the screen face and so that backflushing of liquid through the screen face takes place. While such prior art arrangements are generally useful for thickening and diffusing, they have the following drawbacks; (a) no separation of liquid from suspension takes place during the downstroke of the screen member since backflushing takes place during the downstroke, therefore, the capacity of a screen member of given size is effectively reduced; (b) the backflushing -- in addition to effectively lowering the capacity -- may result in the contamination of a subsequent stage in a multi-stage arrangement where different liquids are diffused into the suspension at different stages; (c) the fiber mat build-up increases during the upstroke of the screen member along the direction of suspension flow so that the rate of separation of liquid decreases during the upstroke, with consequent non-uniform chemical treatment of the suspension when chemical treatment is effected; and (d) relatively large forces are necessary to overcome the friction of the built-up mat during the downstroke, therefore, a fair amount of energy must be used, and the screens must be of relatively heavy construction to maintain their structural integrity once subjected to such large forces.
According to the present invention, all of the above-mentioned disadvantages inherent in the prior art structures, as exemplified by U.S. Pat. Nos. 3,372,087 and 3,563,891, have been overcome. In particular, according to the present invention, no fiber mat build-up takes place on the screen member during any segment of its travel: this results in (1) completely uniform treatment since there is no fiber-mat to effect the rate of flow of liquid through the screen member; (2) no backflushing is necessary to remove a fiber mat (except in rare instances as will be described hereinafter) and thus there is no chance of contamination of the next stage of a multi-stage arrangement; (3) higher capacity can be achieved for a given screen area since there is no wasted time in a downstroke; and (4) since the frictional forces of the screen member relative to the suspension are relatively small because of the absence of a fiber mat, the screen members can be of lighter construction, and the energy requirements for the screen movement can be reduced.
According to the present invention, a method for effective separation of a liquid from a suspension of finely comminuted fiber material and the liquid is practiced comprising the steps of establishing a flow of suspension in a first direction, contacting the flowing suspension with at least one screen member having an apertured face and having a conduit leading away from the face to an area remote from the flow, removing separated liquid from the flow through the conduit, and moving the screen member alternately in the first direction and in the second direction opposite to the first direction; the improvement according to the invention comprises effecting the screen-moving step by continuously oscillating the screen member, with an amplitude larger than the apertures in the screen member face, from a middle position with substantially the same speed in the first and second directions so that fiber mat build-up on the screen face does not take place during movement of the screen face either in the first or second direction. Normally, the apertures in the screen member face are approximately 1/16 inch along the first direction, and the screen member is preferably oscillated with an amplitude of about 1/10 inch. The frequency of oscillation of the screen member is below the natural frequency of the screen member and associated structures, and is at least two cycles per second. Where the speed of flow of the suspension in the first direction is conventional (about 1/10 inch per second), the average speed of oscillation of the screen member in both the first and second directions is about 2.4 inches per second, and the maximum speed in both directions is about 3.8 inches per second for a typical amplitude and frequency of oscillations. Oscillating means effect the oscillating step according to the present invention, and the oscillating means are automatically sensed so that should the oscillating means labor, backflushing of the screen member is automatically effected to eliminate any fiber mat build-up that might have occurred on the screen members to cause the laboring of the oscillating means.
The apparatus according to the present invention comprises a system for effecting separation of the liquid from the suspension of finely comminuted fiber material and the liquid, which suspension is flowing in a first direction, and includes at least one screen member having an apertured face thereof disposed in the flow of suspended fiber material and having a conduit leading away from said face to an area remote from the flow to remove separated liquid from the flow, and apparatus for moving the screen member with respect to the flowing suspension both generally in the first direction and in the second direction opposite the first direction; the improvement according to the present invention consists of the apparatus for moving the screen member comprising means for continuously oscillating the screen member with an amplitude larger than the apertures in the screen member face (the dimensions of the apertures along the first direction), from a middle position with substantially the same speed in both the first and second directions so that fiber mat build-up on the screen face does not take place during movement of the screen face either in the first or second directions. The oscillating means may comprise a synchronous electric motor, and the synchronous electric motor may be connected to the screen member for oscillation of the same through a cam and cam follower assembly. Means may be provided for supporting the majority of the weight of the screen member so that the load on the oscillating means is minimized, and such supporting means may comprise a spring system in operative association with the rod connecting the oscillating means to the screen member. Rotating spray tubes may be provided so that the apparatus can function both as a thickener or a diffuser (bleacher or washer).
It is the primary object of the present invention to provide a method and apparatus for effecting separation of a liquid from a suspension of finely comminuted fiber material and the liquid utilizing a screen member, wherein the fiber mat build-up does not take place on the screen member. This and other objects of the invention will become apparent from an inspection of the detailed description of the invention, and from the appended claims.